The invention relates to a Faraday optical current sensor with polarimetric detection. The present invention further relates to a method of calibrating a current sensor system including a Faraday optical current sensor.
The power industry has a need for monitoring transformer stations for power surges and measurement of large current pulses. For these purposes, a Faraday Effect current sensor has several advantages. A Faraday Effect current sensor may be constructed from dielectric materials, which is of grave importance when measuring at high currents in the presence of substantial electric magnetic interference. Faraday Effect current sensors may employ a coil of an optical fibre or a number of optical fibres, formed of a material exhibiting the Faraday Effect in response to a magnetic field generated by an electric current. A number of prior art patent publications describe Faraday optical current sensors, such as the US publications U.S. Pat. No. 4,894,608, U.S. Pat. No. 5,051,577, U.S. Pat. No. 5,486,754, U.S. Pat. No. 5,811,964, U.S. Pat. No. 6,043,648, all of which are hereby incorporated in the present specification by reference.
In a Faraday effect current sensor, the polarisation plane of a polarised incident light undergoes a rotation, which is a function of the magnetic field created by the electric current to be measured.
The current to be measured can be determined by determining the angle of rotation of the polarization plane of the light on output of the optical sensor. When the light passes through a glass rod the light undergoes a rotation. The angle of rotation may be described by the formula:β=V×B×d 
Where β is the angle of rotation, d is the length of the sensing element, V is the Verdet constant of the glass rod, and B is the magnetic field described as a vector. The Verdet constant is both temperature and wavelength dependent.
In a Faraday Effect current sensor, a light source generates light, which is passed through a polarisation filter or otherwise polarised prior to travelling through the magneto-optical sensitive material. The polarised incident light undergoes a rotation, which is a function of the magnetic field created by the electrical current to be measured. The current to be measured may be determined by metering the angle of rotation of the polarisation plane of the light at the output of the Faraday optical current sensor.
The fibre optic current sensor including the light source and detector is sensitive to, among other things, optical noise in the detection circuit, electrical noise in light source, interference from magnetic fields from nearby inductors and systems, sensor mounting and setup, conductor shape and diameter, sensor production tolerances, temperature effect on Verdets constant, temperature effect on light source and detector, light source and detector degradation over the products lifetime
The determination of the current to be measured is subject to a number of sources of error. Any system based on optics or electrical circuits is sensitive to noise and other signal degradations, such as optical noise or interference from electromagnetic sources. A system comprising a light source, a light detection unit and an optical conduit such as an optical fibre may suffer loss of sensitivity due to optical dampening caused by defects in material used to form an optical conduit or device such as lenses or optical fibres. Light sources and light detection circuitry may be exposed to electrical noise or interference from magnetic fields from nearby inductors or systems, or alternatively to noise caused by fluctuations in the power supply. Also environmental conditions such as temperature have an effect on material properties such as Verdet constant and also an effect on the noise generated in the light source and in the light detection circuitry. Furthermore, all electrical components and light sources and light detection elements suffer degradation over time. All the factors mentioned above contribute to a reduced precision of the measurements performed by the system.